The invention concerns a hydrodynamic torque transmitting unit, particularly a hydrodynamic brake with a toroidal working chamber filled with a working fluid, like that shown in DE-PS No. 976 707, equivalent to GB-PS No. 719,470. In this reference, hydromechanic gearing with hydrodynamic couplings is described. These have a specially designed inlet valve so that, when rapid filling of the working chamber of the coupling is required upon an engagement command, such rapid filling can be effected by first making a large filling cross-section available, and than a small one. However, the large filling cross-section opens only when a control lever is moved rapidly. Otherwise, only the small filling crosssection is opened. Rapid filling of the coupling by means of an initial filling surge requires conscious effort from outside the unit. Furthermore, the time required for the transition from the large to the small filling cross-section depends on the through-flow at a throttle point. Therefore, it differs according to the temperature and the viscosity of the working fluid flowing through the throttle point.
Another known hydrodynamic brake has an extremely sensitive response, whereby the filling of the working chamber takes place within an exceptionally short time after an engagement command. See U.S. Pat. No. 4,194,600. A similar brake is also proposed in U.S. patent application Ser. No. 104,673 filed Dec. 17, 1979. In both cases, there is a large volume inlet control valve which reacts quickly to a control command. However, this valve has only two positions, namely, a closed position and a fully open position. In hydrodynamic brakes of this kind, different levels of braking moment are established by an outlet regulating valve which is separate from the inlet control valve and is arranged in the outlet line. Problems sometimes arise in making the regulation range of the outlet regulating valve sufficiently wide. Therefore, such a brake reacts satisfactorily if moderate, or high or even extremely high braking moment is commanded. However, difficulties arise in establishing a sufficiently low minimum braking moment.
Attempts have been made to solve this problem by not opening the inlet control valve fully when a command is given to establish a low level of braking moment ("Stage I" engagement command), but instead to open this valve, for example, only half way. However, in this instance, the disadvantage arose that the elapsed time between the engagement command and the response of the brake was increased to an unacceptable extent.